1. Field of the Invention:
This invention relates to a method for efficient dehydration of a water-containing substance by electro-osmosis and to an osmotic apparatus for effecting the efficient dehydration. More particularly, this invention relates to novel method and apparatus for efficient dehydration of a water-containing substance by a specific combination of a microporous membrane and a semipermeable membrane.
2. Description of the Prior Art:
Generally, the dehydration of aqueous solution, colloid, paste, dispersion, or gel containing a hydrophilic substance (hereinafter referred to as water-containing substance) has been relatively difficult and has involved an enormous cost. This is paticularly true when the water-containing substance contains a hydrophilic high molecular substance. This is because the water-containing substance often has a high water content due to the peculiar nature of the high molecular substance. Thus, there has been a pressing demand for a method which is capable of efficiently dehydrating such a particularly unmanageable water-containing substance.
For the dehydration of a water-containing substance containing high molecular substances, various methods particularly suited to various properties owned by dissimilar high molecular substances have been proposed or adopted for actual use. For example, the methods of reverse osmosis and ultrafiltration using membranes impermeable to high molecular substances and permeable to water have been proposed for solutions of low viscosity, the methods of reprecipitation such as alcohol precipitation have been proposed for solutions of high viscosity, and the methods of pressing using a gel press and a rubber press have been proposed for hydrous agar, hydrous kappa carragheenin, etc. which gel at normal room temperaure.
Although reverse osmosis and ultrafiltration are efficient methods of dehydration, they are applicable only to those water-containing substances which are solutions of low viscosity and which resist formation of gels and pastes. However, these methods have a disadvantage in that their range of applicability is narrow and that their effective concentration ratio is small. This is because the extent to which the viscosity of the water-containing substances can be lowered by increase of temperature is limited by the durability of the membrane.
The alcohol precipitation method when used for the dehydration of aqueous solutions of high molecular compounds generally necessitates the addition of a large amount of alcohol, when compared to the amount of a high-molecular compound contained in a water-containing substance undergoing treatment. Thus, this method has a disadvantage in that recovery of the spent is a costly procedure.
As methods for the dehydration of highly viscous substances by electro-osmosis, a method for efficient dehydration of gels or pastes has been proposed by Japanese Patent Application Laid-Open Print SHO 54(1979)-76488 and an improved version of this method by Japanese Patent Publication SHO 56(1981)-25167. The method described therein is directed to dehydrating artificially formed gels or pastes by using a cation-exchange membrane of higher electro-osmotic force on the cathode side and an anion-exchange membrane on the anode side, thereby electrically effecting efficient dehydration of a given water-containing substance without requiring mechanical application of pressure. This method also requires installation of at least one water discharge compartment defined on the laterally opposite sides thereof by membranes, which is adapted to protect the water-containing substance undergoing treatment against contamination and to supply required ions to the water-containing substance. This discharge compartment is located in an intermediate portion of the electro-osmosis cell which is not in contact with the two electrodes.
Generally, ion-exchange membranes are made of homogeneous and non-porous materials and exhibit a low electro-osmotic coefficient. In the electro-osmotic dehydration of a water-containing substance, the amount of water permitted to pass through the ion-exchange membranes cannot be overly large, due to the amount of electric power required.